Deploy binddns-auth BIND cluster

Adds the authoritative masters (pod-0 primary + 2 secondaries via catalog/AXFR); includes the catalog zone and transfer TSIG key. Stacked on the bind-operator deploy so the
CRDs and kubeconform schemas are present; merge the operator PR first.

- add apps/base/binddns-auth and its au-syd1 overlay

apps/base/binddns-auth/cluster.yaml

@@ -0,0 +1,36 @@
+---
+# Authoritative masters (replaces the 3x Puppet authoritative servers).
+# pod-0 is the primary; pods 1-2 replicate via the catalog zone + AXFR/IXFR.
+apiVersion: bind.unkin.net/v1alpha1
+kind: BindCluster
+metadata:
+  name: auth
+  namespace: binddns-auth
+spec:
+  mode: authoritative
+  replicas: 3
+  storageClassName: cephrbd-fast-delete
+  storageSize: 2Gi
+  service:
+    type: LoadBalancer
+    annotations:
+      purelb.io/service-group: common
+      external-dns.alpha.kubernetes.io/hostname: ns-auth.k8s.syd1.au.unkin.net
+  resources:
+    requests:
+      cpu: 100m
+      memory: 128Mi
+    limits:
+      cpu: "1"
+      memory: 512Mi
+---
+# Catalog zone so new BindZones auto-provision onto the secondaries.
+apiVersion: bind.unkin.net/v1alpha1
+kind: BindCatalogZone
+metadata:
+  name: auth-catalog
+  namespace: binddns-auth
+spec:
+  clusterRef: auth
+  zoneName: catalog.internal
+  transferKeyRef: transfer-key